When plant tissue is abiotically injured by crushing, cutting,
flaming, electrical stimulation or several other means, the injured (perceiving) tissue generates electrical
signals, including action potentials and variation potentials. These are transmitted to distant regions
(responding tissue) where they evoke apparently disparate responses, including callose formation, closing of
plasmodesmata, stoppage of cytoplasmic streaming, inhibition of ribosome movement along mRNA, and ultra-rapid
but transient accumulation of several hundred transcripts, which are degraded without being translated. These
apparently disparate responses can be reconciled by one fundamental hypothesis that assumes that “the
plant does now know what hit it” and thus “expecting the worst” mounts a holistic defense
response against its most potent nemesis, a putative viral invasion.
The basis for this response
is calcium influx into the cytoplasm via voltage-gated channels (action potential) associated with the
microtubules, or via mechano-sensitive channels (variation potential) associated with microfilaments. The
calcium interacts with calcium and/or calmodulin-dependent cytoskeleton-associated protein kinases. This
causes the phosphorylation of myosin, which stops cytoplasmic streaming, and of elongation factor 2F, which
slows elongation and causes ribosomes to pile up on polysomes, thereby decreasing protein synthesis, but
protecting pre-existing “host” transcripts from degradation. The phosphorylation signal then
passes into the nucleus where it phosphorylates RNA polymerase II, which goes into overdrive, (i.e., does not
stop at accuracy check-points), thus causing the synthesis of large amounts of mis-made mRNA. The mRNA is
transported into the cytoplasm where it is scanned (checked for accuracy) by ribosomes, and found to be
incorrect. This surveillance mechanism stimulates ribonuclease activity which degrades the free
(non-polysome-associated), mis-made RNA, but leaves the original, “host” mRNA unscathed since it
is protected by ribosomes. The ribonuclease also (and here is the crux of the matter) attack other free mRNAs,
including viral mRNAs, so these are disposed before they can be translated. Within minutes this reaction is
over, cytoplasmic steaming resumes, translation continues, ribosomes are released and so can be used to
translate new (correctly-made) transcripts. |